Exchange-Induced Upconversion in Rb2MnCl4:Yb3+

Abstract

Photon upconversion from the near-IR (NIR) to the visible (VIS) is potentially interesting for many applications. The wellknown process of second-harmonic generation in nonlinear optical materials is widely used for the conversion of NIR laser radiation into VIS or UV radiation. It requires coherent radiation. In this paper we are talking about a different nonlinear upconversion (UC) process, which does not require coherent radiation. Instead, it requires the existence of a metastable intermediate state in the NIR, which is reached with the first photon. The second photon then promotes the system to a higher excited state, followed by emission from a second metastable state in the VIS. The first UC step can be followed by a second and third UC step, if there exist metastable states at higher energies. Since the early reviews by Auzel 1 and Wright 2 about UC processes, there has been a lot of work on UC processes and UC materials. Both fundamental studies of new materials and the mechanisms of upconversion and investigations of the application potential of the UC process have been reported. These potential applications are in the areas of lighting, lasers, displays, and imaging systems as well as NIR photon counters. 3-5 Most of this work is based on lanthanide-doped crystals and glasses. f-f excited states of lanthanides are often metastable because of the shielding of the f-electrons and the resulting small electron-phonon coupling. Er 3+ ,T m 3+ ,N d 3+ , and Ho 3+ are